Clean up CSS for Pholio upload thing

Summary: Ref T3572. Pure JS/CSS changes, just cleaning up some of the mess I made and slightly improving the behavior on mobile (you won't be able to edit images on mobile, but you could fix descriptions and titles, at least).

Test Plan: {F50887}

Reviewers: btrahan, chad

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T3572

Differential Revision: https://secure.phabricator.com/D6500